1. Field of the Invention:
This invention relates to drive devices for cameras in which film winding and other operations are motorized by respective drive sources.
2. Description of the Prior Art:
In the past, the drive device having a single motor by which charging of the various portions of the camera and film winding are performed was poor in efficiency because a large number of gear trains are necessary to transmit the driving torque of the motor to charge the various portions of the camera which are widely different in positions. Therefore, a high speed charging was realized only by increasing the number of batteries at a time.
To improve this, it is considered to use two motors for charging the various portions of the camera and winding up film respectively in combination with respective transmission systems whose speed reduction ratios are set to optimum values, thereby giving an advantage that the charging speed can be increased with the limitation of the number of batteries to a minimum.
In such a case, the two motors must be initialized at the same time soon after the shutter is closed. Or otherwise, the charging and the winding could not be completed in a shortest time. In the early stage of current supply to the motor, however, overcurrent (rush current) of intensity dependent on the internal resistance of the motor is allowed to flow. Such a sudden outflow of large current from the battery results in very large a drop of the battery voltage due to the internal resistance of the battery itself. Therefore, the coincidence of the starts of energization of the two motors leads to a high possibility for the battery voltage to fall below the minimum satisfactory operating level for a microcomputer or like control means. Thus, the operation of the microcomputer will often be troubled.
Meanwhile, the conventional motorized winding device was made also serve to charge the various portions of the camera by using a common drive source of the film winding-up system. For this reason, each time the camera was charged, the film winding-up system, too, was driven to move regardless of whether or not the camera was loaded with a film cartridge in a chamber therefor. Recently there has been proposed a camera which enables the efficiency of charge to be increased, electrical energy to be saved, and the winding period to be shortened, by using separate motors assigned to the film winding and the camera charging respectively. But, even in such a camera, the wind-up-motor and the charge motor have a common current supply circuit. Therefore, the circuit has, despite the camera is unloaded, to supply current to the wind-up motor too. As the wind-up motor is associated with a driving torque transmission of large mass, large wasteful consumption of electrical energy and fruitless lowering of durability resulted.
In this connection it should be noted that if the current supply circuit is made responsive to unloading of the camera, it becomes impossible to check the motorized functions of the camera in advance of its use, because all the motors do not work. Also, if the photographer pushes down the release button to check the operation of the wind-up motor under the condition that the back cover of the camera is open, the release of the shutter is not followed by the operation of the drive sprocket for advancing film or the take-up spool. So he will mistake it for occurrence of a malfunction of the camera.
Also, whilst the film winding was motorized, to advance the film by one frame still relied on the mechanism that had so far conventionally been employed in the manually operated cameras. This mechanism arrests the driving torque transmission between the motor and the take-up spool in response to detection of when the film has advanced one frame. In such a motorized camera, the current supply to the motor was cut off by using a mechanical switch arranged to open when the transmission is arrested.
Though the use of such a one-frame indexing mechanism is prerequisite for the manually operated cameras, it is not always necessary to the motorized cameras. So, in recent years, omission of the aforesaid mechanism from the motorized camera has alreadly been made.
Then, instead of using the mechanism, the advancement of the film through the length of one frame is controlled by adjusting the period of energization of the motor for each shot. In order to rapidly stop the motor with the help of braking means, both ends of winding of the motor may be either short-circuited, or reversely biased. For this purpose, it is, however, required that even after the termination of energization of the motor, a set of transistors controlling the current supply to the motor be continued operating by sustaining the flow of base current to them not only when the braking method of supplying reverse current to the motor is employed, as a matter of course, but also when another method of short-circuiting the ends of winding of the motor is employed. To fulfill this requirement, a drawback has been produced that, as the rate of electrical power consumption per shot increases, the life-time of the electrical power source or battery is very shortened. For this reason, it is desired to minimize the braking period of the wind-up motor as far as possible.
Meanwhile, the motorized cameras have generally the continuous shooting mode. In order to increase the firing rate of shots for a high speed shooting, the initiation of a shutter operation must be brought into as near coincidence with the termination of the film winding operation as possible. In the past, this techniqe has not been compatible with the above-described measure of minimizing the braking period, because the removal of the braking action from the wind-up motor allows for later slight movement of the film due to the elasticity of film itself and the relax of the gear train from the stress of transmitting the film to occur during the next exposure. Thus, a problem has been produced that blurred images are photographed.